patch 8.1.0679: sign functions do not take buffer argument as documented

Problem:    Sign functions do not take buffer argument as documented.
Solution:   Use get_buf_tv(). (Yegappan Lakshmanan, closes #3755)
diff --git a/src/evalfunc.c b/src/evalfunc.c
index 97921e7..a46b339 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -11365,7 +11365,7 @@
     if (argvars[0].v_type != VAR_UNKNOWN)
     {
 	// get signs placed in this buffer
-	buf = find_buffer(&argvars[0]);
+	buf = get_buf_tv(&argvars[0], FALSE);
 	if (buf == NULL)
 	{
 	    EMSG2(_("E158: Invalid buffer name: %s"),
@@ -11457,7 +11457,7 @@
 	goto cleanup;
 
     // Buffer to place the sign
-    buf = find_buffer(&argvars[3]);
+    buf = get_buf_tv(&argvars[3], FALSE);
     if (buf == NULL)
     {
 	EMSG2(_("E158: Invalid buffer name: %s"), tv_get_string(&argvars[2]));
@@ -11566,7 +11566,7 @@
 
 	if ((di = dict_find(dict, (char_u *)"buffer", -1)) != NULL)
 	{
-	    buf = find_buffer(&di->di_tv);
+	    buf = get_buf_tv(&di->di_tv, FALSE);
 	    if (buf == NULL)
 	    {
 		EMSG2(_("E158: Invalid buffer name: %s"),